Abstract

The La Vérendrye Park area extends from the Grenville front near Val d'Or, Quebec, to the beginning of the cover formed by rocks of the Grenville Supergroup some 200 km southeast of the front. Most of the rocks in the study area are layered quartzofeldspathic gneisses of uncertain origin, metamorphosed to the granulite facies.

For the first 50 km southeast of the front, the gneisses yield fairly precise Rb-Sr isochron estimates of the times of their metamorphism 2,450 to 2,850 m.y. ago. Throughout the central part of the area, lithologically similar rocks have commonly been affected by a later metamorphic event; they yield a variety of results ranging from a random distribution of isotopic data to very poorly defined ages of about 2,000 m.y., which are interpreted as being intermediate between the ages of two metamorphic episodes.

Demonstrably intrusive igneous rocks are very rare in the northern and central parts of the area. A metasomatic origin for the fairly abundant granitic (microcline-rich) layers in the gneisses is supported by isotopic and petrographic evidence and by their relationship to the enclosing quartz-oligoclase gneisses. Similar evidence suggests that this granitization is an Archean phenomenon in the northern half of the area.

Farther south, the zone of migmatite between 150 and 200 km southeast of the Grenville front consists of granitic and charnockitic rocks that yield ages of about 1,150 m.y. and locally high initial 87Sr/86Sr ratios. The host rocks are interfolded Archean and Aphebian quartzofeldspathic gneisses. The latter are a group of metasedimentary rocks that are distinct in lithology from the Archean gneisses and the sedimentary rocks of the Grenville Supergroup and yield a reliable metamorphic age of 1,650 m.y. and a low initial 87Sr/86Sr ratio.

When this information is added to that from other more southern regions, the evolution of this part of the shield is inferred to be dominantly accretionary but includes a very significant overlap of Grenville Supergroup rocks on Aphebian continental crust. The Grenville front in this area probably represents a zone of vertical movement that exhumed a 100-km-wide strip of high-grade Archean rocks, which presumably had lain under and adjacent to the Abitibi foldbelt. South of the La Vérendrye Park area, Archean rocks have not been identified in close association with rocks of the Grenville Supergroup, and older rocks have not contributed significantly to the generation of plutonic rocks of Grenville age except in the boundary zone described in this paper.

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